The present invention relates to a hologram apparatus for performing volume recording of a hologram on a hologram recording material, and particularly to a hologram recording method for easily performing tracking servo at the time of reproduction.
In recent years, there have been proposed holographic storage systems for recording/reproduction of a large capacity of data by utilizing the hologram technology. The holographic storage systems include a recording system in which a hologram recording material (hereinafter sometimes referred to simply as recording material) is irradiated, at predetermined angles, with both a signal beam including recording data generated by space light modulation means such as a liquid crystal device and a reference beam set according to the signal beam so that an interference fringe generated by the signal beam and the reference beam is recorded on the recording material, and a reproduction system in which the hologram recording material is irradiated with a reproduction illumination beam to generate a diffracted beam (reproduced signal beam) corresponding to the recorded interference fringe, and the diffracted beam is received by a light receiving device such as a CCD image sensor and analyzed to reproduce the data. Incidentally, the hologram thus recorded per space light modulation means is called a page.
In addition, in the holographic storage system, a technique called multiplex recording is used for enhancing the recording density. Unlike the recording on an optical disk in the past, the multiplex recording resides in recording a multiplicity of independent pages in a single location. Representative known examples of the multiplex recording system include angle multiplex recording system, shift multiplex recording system, phase code multiplex recording system, and other many multiplexing systems.
The angle multiplexing system records and reproduces a multiplicity of independent pages in a single location by varying the angle of the reference beam. The shift multiplexing performs multiplex recording by shifting the recording position little by little. The phase code multiplexing records data by simultaneously radiating the reference beams from various directions in recording one page, while the reference beams from various directions are provided with phase variations. The angle multiplexing system records and reproduces a multiplicity of independent pages in a single location by combining the phase variations in various ways.
Besides, many multiplexing systems are known in addition to the above three kinds. For example, the systems pertaining to the present application include a system called speckle multiplexing (or correlation multiplexing) described, for example, in Holographic Data Storage; H. J. Coufal, D. Psaltis, G. T. Sincerbox ED; Springer; p. 47 Volume Holographic Multiplexing Methods, which will be described in detail below.
The speckle multiplexing is a method in which a diffusing body, for example, is placed in the optical path of the reference beam, and the reference beam diffused by the diffusing body and the signal beam are brought into interference with each other in a recording material. Generally, a laser beam having undergone random diffusion has, as a result of random interference, a random intensity distribution called a speckle pattern. Namely, in the speckle multiplexing system, the speckle pattern and the signal beam interfere with each other. The hologram recorded in this manner is characterized in that the image would not be reproduced after a displacement by a distance equivalent to the speckle size. The multiplexing method utilizing this characteristic feature is the speckle multiplexing (or correlation multiplexing), which has the merit that it is possible to record in a higher density (hence, to be better in shift selectivity) than in shift multiplexing using a spherical wave.
For example, shift recording as shown in FIG. 12 may be conducted by utilizing a hologram based on the speckle multiplexing system. For easy understanding of the discussion, it is assumed that the recording is conducted on a rotating disk type recording material such as an optical disk and that the shifting direction coincides with the rotating direction of the disk. Reading is also conducted while rotating the disk. In this figure, one circle represents one hologram page of recording area. However, the shape of the recording area is not limited to the circle and may be an ellipse or a rectangle. Reproduction of the hologram thus recorded will be considered. Incidentally, the holograms do not overlap in the orthogonal direction (cross-track direction), they may overlap in that direction.
In the optical disks in the past, the shifting direction is also called the track direction, while the direction orthogonal to the shifting direction is called the cross-track direction, and the servo against the crosswise deviations of the position in the track direction attendant on the rotation of the disk (i.e., tracking servo) has had an important role in reproduction. Incidentally, deviation amounts of several tens to several hundreds of micrometers have been permitted in the cases of the optical disks in the past.